Xycom XVME-653, Manual

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XVME-653/658 Manual
4-14
Numeric Consistency
Numeric consistency, or data consistency, refers to communications between the
XVME-653/658 and the VMEbus in which the byte-ordering scheme described above is
maintained during the transfer of a 16-bit or 32-bit quantity. Numeric consistency is
achieved by setting the XVME-653/658 buffers to pass data straight through, which al-
lows the Universe chip to perform address-invariant byte-lane swapping. Numeric con-
sistency is desirable for transferring integer data, floating-point data, pointers, etc. Con-
sider the long word value 12345678h stored at address M by both the XVME-653/658
and the VMEbus, as shown in Figure 4-3.
12 34 56 78
78
56
34
12
12 34 56 78
12
34
56
78
Pentium Register (32 bit)
VMEbus
Byte-swapping
Hardware
Address
M
M+1
M+2
M+3
XVME-653/658 VMEbus
Figure 4-3 Maintaining Numeric Consistency
Due to the Universe chip, the data must be passed straight through the byte-swapping
hardware. To do this, maintaining numeric consistency, enable the straight-through buff-
ers by setting bits 6 and 7 of the NVRAM and DiskOnChip Port (Register 234h) to 1 (see
p. 2-6).
Note
With the straight-through buffers enabled, the XVME-653/658 does not
support unaligned transfers. Sixteen-bit or 32-bit transfers must have an
even address.